Method for making a seam-sealable non-magnetic lid and package

ABSTRACT

A non-magnetic lid for sealing a hermetic package. The lid includes a molybdenum substrate having a sputtered adhesion layer and a copper seed layer. The lid also includes a plated palladium solder base layer, and has a gold/tin solder preform attached to a sealing surface of the lid.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a divisional of, and claims priority to, U.S.patent application Ser. No. 14/529,410, filed Oct. 31, 2014, thedisclosure of which is incorporated herein by reference in its entirety.

BACKGROUND

Technical Field

Embodiments of the invention relate generally to packaging of componentsthat are sensitive to temperature, humidity, and/or atmosphericconstituents. Particular embodiments relate to lids suitable forhermetically sealing packages that contain sensitive components such asmicro-electrical-mechanical systems (“MEMS”) that are suitable for usein magnetic-resonance imaging (“MRI”) equipment and in otherapplications that require non-magnetic components.

Discussion of Art

MEMS are devices that have their largest dimension within a range fromabout 20 micrometers to about 1 millimeter (0.02-1.0 mm). These verysmall electrical machines are useful in many applications, for example:ejecting ink from the cartridges of inkjet printers to put letters ontoa page; measuring accelerations of a vehicle or of a handheld devicesuch as a cellular phone or a game controller; transducing air pressurewaves or surface vibrations to record sound; switching optical signalsamong fiber arrays; etc.

Generally, MEMS are useful for reliably providing highly responsive(small time constant) electromechanical functionality, such as motionsensing, within a small footprint or volume envelope. Accordingly, ithas been desired for several years to make use of MEMS for sensing andcontrol within MRI systems. However, it is necessary in MRI systems toprovide components that are “non-magnetic,” i.e., neither ferromagneticnor paramagnetic.

To date, MEMS packaging has relied upon materials that are eitherferromagnetic and/or paramagnetic. This has been the case in partbecause packages fabricated from non-magnetic materials, e.g., ceramicsor plastics, have been understood to require closure methods that riskedthermal damage, e.g., brazing, and/or chemical damage, e.g., volatile ormoisture-permeable adhesives, to the MEMS enclosed within the packages.MEMs are sensitive to their environment, particulate or chemically, andalso are sensitive to the packaging processing conditions, thus the needfor a process to control the conditions and package environment. Indeed,MEMS typify a category of “sensitive” components that require specialcare in packaging. Other constituents of this category may includepiezoelectric, paramagnetic, and shape memory alloy devices.

In view of the above, it is desirable to provide a sensitive componentwithin a non-magnetic hermetic package. A key difficulty in providingsuch a package has been to devise a method of bonding a sensitivecomponent into an open non-magnetic package cavity, then sealing thepackage cavity, without inducing thermal and/or chemical damage to thesensitive component.

BRIEF DESCRIPTION

Embodiments of the invention provide a non-magnetic lid for sealing ahermetic package. The lid includes a molybdenum substrate, a sputteredadhesion layer, a sputtered copper seed layer, and a plated palladiumsolder base layer, as well as a gold/tin solder preform attached at asealing surface of the lid.

Aspects of the invention provide for making a non-magnetic seam-sealablelid for sealing a non-magnetic hermetic package, according to a methodthat includes forming a lid-shaped substrate from a molybdenum blank;sputtering onto the substrate an adhesion layer that includes, orconsists essentially of, one or more materials such as: titanium,tantalum, or chromium; sputtering onto the adhesion layer a copper seedlayer; electroplating onto the seed layer a palladium solder base layer;and attaching a gold/tin solder preform at a sealing surface of the lid.

Other aspects of the invention provide a method for making anon-magnetic MEMS package, which includes bonding a MEMS device within anon-magnetic package body that includes a sealing edge defined by anon-magnetic metallic seal ring, and sealing to the sealing edge of thepackage body a non-magnetic lid having a sealing surface that includes agold/tin solder preform, which is sealed to the sealing edge of thepackage body.

Yet other aspects of the invention provide a metallic non-magnetic lidfor sealing a hermetic package that includes a molybdenum substrate, aphysical vapor deposited adhesion layer, a copper seed layer, apalladium solder base layer, and a gold/tin solder preform attached at asealing surface of the lid.

DRAWINGS

The present invention will be better understood from reading thefollowing description of non-limiting embodiments, with reference to theattached drawings, wherein below:

FIG. 1 is a perspective partially sectioned view of a non-magnetic lidaccording to an embodiment of the invention.

FIG. 2 is a top view of a non-magnetic package body.

FIGS. 3A-3C are a perspective view of a glove box and detailed views ofa tacker and seam sealer housed in the glove box.

FIG. 4 is a partial sectioned view of an embodiment of the inventionattached to the non-magnetic package body of FIG. 2.

DETAILED DESCRIPTION

Reference will be made below in detail to exemplary embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference characters usedthroughout the drawings refer to the same or like parts, withoutduplicative description. Although exemplary embodiments of the presentinvention are described with respect to their use in MRI systems,embodiments of the invention are applicable for use in any setting thatcan benefit from non-magnetic packaging.

As used herein, the terms “substantially,” “generally,” and “about”indicate conditions within reasonably achievable manufacturing andassembly tolerances, relative to ideal desired conditions suitable forachieving the functional purpose of a component or assembly.

Referring generally to FIGS. 1-4, in an exemplary embodiment, aseam-sealable non-magnetic lid 100, as shown in FIG. 1, is provided forsealing a non-magnetic MEMS package body 200, as shown in FIG. 2. Thelid 100 is sealed onto the non-magnetic MEMS package body 200 by using aseam sealer 300, disposed within a dry air glove box 310, as shown inFIG. 3A. The seam sealer 300 applies electrical current from itselectrodes 302 progressively to lid surface 100 and to a patternedmetallic seal ring 250 of the package body 200, thereby locallyreflowing together the lid with a gold/tin sealing preform 150 with themetallic seal ring 250. Completion of the seam sealing process producesa hermetic non-magnetic MEMS package 400, shown in perspective in FIG.3C and shown as a partial sectioned schematic in FIG. 4.

As used herein, “hermetic” means obtaining a helium leak rate of 3×10^−8mBarr L/s or less using a helium fine leak tester (e.g., an Alcatel™ASM180 model) after two hours hold under at least 60 psi helium; andshowing no bubbles in 125 C solution of FC-40; or equivalent testresults.

Referring now to FIG. 1, the lid 100 includes a molybdenum substrate 110that is shaped as the lid. For example, the substrate 110 may be stampedor cut to a flat shape, or it may be stamped or drawn to a concave orstepped shape. The substrate is at least about 5 mil (0.005 inch) thick,usually about 10 mil thick, but can be thicker. Generally, a minimumthickness for the substrate 110 is set by a desire to maintain dry airat atmospheric pressure within the package 400, even duringdepressurized air cargo transport, without permanent deformation of thesubstrate. The molybdenum substrate also desirably provides for thermaland electrical resistivities that facilitate the seam sealing process.

The lid 100 also includes, over the molybdenum substrate 110, anadhesion layer 120 that is at least about 500 angstrom thick and can beup to about 2000 angstrom thick. The adhesion layer can be sputteredonto the substrate 110 from titanium or from similarmolybdenum-compatible metals such as tantalum or chromium. In certainembodiments, equivalent methods of physical vapor deposition (PVD) canbe used in place of sputtering. PVD processes include cathodic arc,electron beam, resistive evaporative, pulsed laser, and magneticsputtering deposition techniques. In certain embodiments, magneticsputtering is the coating method, primarily due to chemical difficultiesin electroplating or chemical vapor deposition (CVD). A sputteredadhesion layer has been found more uniform and provide better adhesionthan a plated or CVD adhesion layer, and sputter systems provide backsputter capabilities to improve adhesion further. Other PVD techniquesare believed equally suitable by comparison to electrochemical or CVDtechniques. The adhesion layer 120 is sputtered onto opposite broadsurfaces of the substrate, and is sputtered at least thick enough thatthe edges of the sputtered regions join to cover the edges of thesubstrate; thus, minimum acceptably thickness of the adhesion layer willvary according to thickness of the substrate. The adhesion layer 120generally is not much thicker than needed to cover the edges of thesubstrate from the two broad surfaces.

Over the adhesion layer 120, the lid has a copper seed layer 130 that issputtered to at least about 1000 angstrom thick. Copper has been foundsuitable for the seed layer 130 in terms of its compatibility with thetitanium/tantalum/chromium adhesion layer, and in terms of itsacceptance for electroplating palladium. The seed layer 130 is notdirectly sputtered onto the molybdenum because the adhesion layer 120has been found to greatly enhance attachment of the seed layer 130 tothe substrate 110. In some embodiments the seed layer 130 may be about2000 angstrom thick, and in certain embodiments the seed layer may be asmuch as about 6000 angstrom thick. A thicker seed layer helps to bufferback-etch from a subsequent step of electroplating, and helps to coveredges of the substrate and allows easy plating of the palladium onto thecopper without additional plating steps. On the other hand, increasedthickness adds cost and weight, and an excessive thickness of copper,which is highly thermally and electrically conductive, may presentproblems with a later step of seam-sealing the lid to close the MEMSpackage.

Outside the seed layer 130, the lid 100 includes a palladium solder baselayer 140 that is about 1-2 μm thick, where again the thickness ischosen to ensure that edges of the substrate 110 are covered. Thepalladium solder base layer 140 typically is electroplated onto thecopper seed layer 130. Accordingly, the palladium solder base layer 140will cover all surfaces of the lid 100 so as to mitigate tarnishing ofthe copper seed layer. In other embodiments, the solder base layer 140may be sputtered onto just one surface (a sealing surface 104) of thelid 100. However, conformality of the solder base layer onto the edgesof the lid 100 is helpful in order to enhance sealing action of thesolder preform, as further discussed below. In certain embodiments, agold strike layer 145 may be electroplated between the seed layer 140and the solder base layer 130, in order to enhance attachment of thesolder base layer to the seed layer.

On the sealing surface 104 of the lid 100, near its edges, the gold/tinsolder preform 150 then is attached to the solder base layer 140, e.g.,by tack welding. Overall, the materials and various layer thicknesses ofthe lid 100 are chosen so that the lid has bulk resistivity sufficientlyhigh to focus current for spot seam sealing of the solder preform 150,as opposed to welding the lid itself. Thus, the inventive lid 100 allowsfor a low power seam sealing process that does not impose bulk heatingof the lid or of a structure to which the lid is attached. Therefore, itis possible to use the lid for hermetic sealing of a MEMS package,without risking thermal damage to a MEMS device within the package. Thelid 100 also may include a gold flash layer 160 between the palladiumsolder base layer 140 and the solder preform 150.

Referring now to FIGS. 2 and 4, an example of a MEMS package body 200,suitable for closure by the lid 100, includes a wall 201 that is toppedby the generally planar metallic seal ring 250. The seal ring 250thereby defines a sealing edge of the package body. The wall 201encloses a cavity 204. Within the cavity 204, a MEM(micro-electro-mechanical) device 210 is bonded to the package body 200.

Referring again to FIG. 3, the steps of tacking and seam sealing areaccomplished within a glove box 310, which maintains clean air under dryconditions (less than about −40° C. dew point, 0% relative humidity,less than about 10,000 ppm water vapor). During tacking, the lid 100 isplaced onto the package body 200 and the two components are placedtogether into a tack-welder 320 within the glove box. The tack welder320 is used to lightly press down the lid 100 and to tack the lid withthe gold/tin solder preform 150 onto the metallic seal ring 250 at oneor two locations. Before tacking, the lid 100 and the package body 200may be O2 ashed/cleaned and then purged with CDA (clean dry air asdiscussed below) and heated in a vacuum oven 330, which is connectedwith and opens into the glove box 310. For seam sealing, the packagebody 200 is transferred within the glove box onto a rotatable stage 304of the seam sealer 300.

The seam sealer 300 then is operated to rotate the stage 304 with thelid and body, while retracting and extending the electrodes 302, so asto move the electrode contact along the edge 102 of the lid 100 that thepreform 150 and the seal ring 250 will be progressively locally heatedand welded to form an hermetic weld fillet between the solder base layerand an opposed seal ring. In this regard it is helpful to have the edgesof the substrate 110 fully covered by the solder base layer 140 andsubordinate layers 120, 130, so as to form a uniform fillet 450 thatgoes up the side edge of the lid 100, as shown in FIG. 4. Additionallyit is helpful to have the lid and package body tacked together at onlyone location, where the electrodes 302 will start the seam-sealingprocess.

According to an aspect of the invention, seam sealing, which is alocalized heating process, is usable instead of brazing (which is wellknown for sealing solder preforms). Although brazing has beenconventional for sealing packages such as the inventive package 400, itturns out that the generalized high temperatures obtained during brazingcan damage and deform the MEMS device that is meant to be enclosedwithin the hermetic MEMS package article. In order to implement seamsealing, however, it is necessary that the lid 100 must enable localizedheating. Many non-magnetic metals make it very difficult to provide heatonly in a local area, because such metals tend to be highly thermallyand/or electrically conductive. Therefore, the materials and layerthicknesses of the lid 100 are carefully selected in order to optimizeits thermal and electrical properties for seam sealing, which usesresistance welding—to locally heat and melt the preform 150.

Processes as described above result in an hermetic and non-magnetic MEMSpackage 400, which is usable in various applications including withinMRI enclosures. Because dry air at atmospheric pressure is capturedinside the package 400, it also may be usable in other environmentswhere MEMs have not previously been considered.

Embodiments of the invention therefore provide a non-magnetic lid forsealing a hermetic package. The lid includes a molybdenum substrate, asputter deposited adhesion layer, a copper seed layer, and a palladiumsolder base layer, as well as a gold/tin solder preform attached at asealing surface of the lid. The adhesion layer may be sputtered. Thesubstrate may be back-sputtered, chemically etched, and/or mechanicallyetched prior to sputtering the adhesion layer. The solder base layer maybe at least about 10,000 angstrom (1 micron) thick. In some embodiments,the solder base layer may be no more than about 2 micron (2 μm) thick.The lid also may include, over the solder base layer an electroplatedgold coating to about 1000 angstrom—2 μm thick. The lid may be vacuumbaked prior to attaching the gold/tin preform. For example, the lid maybe vacuum baked to achieve less than about 6 ppm H concentration in thepalladium solder base layer, thereby reducing H available to diffusefrom the solder base layer into a package to be sealed by the lid. Thelid may have at its outside edges a corner radius of about 30 mil. Theadhesion layer may be at least about 500 angstrom thick. The adhesionlayer may be about 2000 angstrom thick. The seed layer may be at leastabout 1000 angstrom thick, or about 2000 angstrom thick. In someembodiments, the seed layer may be no more than about 6000 angstromthick. In some embodiments, the adhesion layer may include one or morematerials such as: titanium, tantalum, or chromium. For example, theadhesion layer may include, or may consist essentially of, titanium. Theadhesion layer, the seed layer, and the solder base layer may cover atleast a sealing surface of the substrate and an edge of the substratethat bounds the sealing surface. In some embodiments, the adhesionlayer, the seed layer, and the solder base layer uniformly cover thesubstrate.

Aspects of the invention provide for making a non-magnetic seam-sealablelid for sealing a non-magnetic hermetic package, according to a methodthat includes forming a lid-shaped substrate from a molybdenum blank;physical vapor depositing (e.g., sputtering) onto the substrate anadhesion layer that includes, or consists essentially of, one or morematerials such as: titanium, tantalum, or chromium; physical vapordepositing (e.g., sputtering) onto the adhesion layer a copper seedlayer; electroplating onto the seed layer a palladium solder base layer;and attaching a gold/tin solder preform at a sealing surface of the lid.The method also may include at least one of back sputtering, chemicallyetching, or mechanically etching the substrate prior to sputtering theadhesion layer. The method also may include vacuum baking the lid priorto attaching the gold/tin solder preform, e.g., vacuum baking to lessthan about 6 ppm H concentration in the solder base layer. The adhesionlayer may consist essentially of titanium.

Other aspects of the invention provide a method for making anon-magnetic hermetic package, which includes bonding a sensitivecomponent within an interior cavity defined by non-magnetic walls thathave a sealing edge defined by a non-magnetic metallic seal ring, andsealing to the sealing edge a non-magnetic lid, which has a sealingsurface that includes a gold/tin solder preform, which is sealed to thesealing edge. The lid may include a molybdenum substrate, a physicalvapor deposited adhesion layer that includes a material such astitanium, tantalum, or chromium, a copper seed layer, and/or a palladiumsolder base layer.

Yet other aspects of the invention provide a metallic non-magnetic lidfor sealing a hermetic package that includes a molybdenum substrate, aphysical vapor deposited adhesion layer, a copper seed layer, apalladium solder base layer, and a gold/tin solder preform attached at asealing surface of the lid.

It is to be understood that the above description is intended to beillustrative, and not restrictive. For example, the above-describedembodiments (and/or aspects thereof) may be used in combination witheach other. In addition, many modifications may be made to adapt aparticular situation or material to the teachings of the inventionwithout departing from its scope. While the dimensions and types ofmaterials described herein are intended to define the parameters of theinvention, they are by no means limiting and are exemplary embodiments.Many other embodiments will be apparent to those of skill in the artupon reviewing the above description. The scope of the invention should,therefore, be determined with reference to the appended claims, alongwith the full scope of equivalents to which such claims are entitled. Inthe appended claims, the terms “including” and “in which” are used asthe plain-English equivalents of the respective terms “comprising” and“wherein.” Moreover, in the following claims, terms such as “first,”“second,” “third,” “upper,” “lower,” “bottom,” “top,” etc. are usedmerely as labels, and are not intended to impose numerical or positionalrequirements on their objects. Further, the limitations of the followingclaims are not written in means-plus-function format and are notintended to be interpreted based on 35 U.S.C. § 112, sixth paragraph,unless and until such claim limitations expressly use the phrase “meansfor” followed by a statement of function void of further structure.

This written description uses examples to disclose several embodimentsof the invention, including the best mode, and also to enable one ofordinary skill in the art to practice embodiments of the invention,including making and using any devices or systems and performing anyincorporated methods. The patentable scope of the invention is definedby the claims, and may include other examples that occur to one ofordinary skill in the art. Such other examples are intended to be withinthe scope of the claims if they have structural elements that do notdiffer from the literal language of the claims, or if they includeequivalent structural elements with insubstantial differences from theliteral language of the claims.

As used herein, an element or step recited in the singular and proceededwith the word “a” or “an” should be understood as not excluding pluralof the elements or steps, unless such exclusion is explicitly stated.Furthermore, references to “one embodiment” of the present invention arenot intended to be interpreted as excluding the existence of additionalembodiments that also incorporate the recited features. Moreover, unlessexplicitly stated to the contrary, embodiments “comprising,”“including,” or “having” an element or a plurality of elements having aparticular property may include additional such elements not having thatproperty.

Since certain changes may be made in the above-described article andmethod of making the lid, without departing from the spirit and scope ofthe invention herein involved, it is intended that all of the subjectmatter of the above description or shown in the accompanying drawingsshall be interpreted merely as examples illustrating the inventiveconcept herein and shall not be construed as limiting the invention.

What is claimed is:
 1. A method for making a seam-sealable non-magneticlid, the method comprising: forming a lid substrate from a blankconsisting of molybdenum; depositing an adhesion layer onto the lidsubstrate; forming a copper seed layer on the adhesion layer; forming apalladium solder base layer on the copper seed layer; and coupling agold/tin solder preform to the palladium solder base layer.
 2. Themethod of claim 1 further comprising at least one of back sputtering,chemically etching, or mechanically etching the lid substrate prior tosputtering the adhesion layer.
 3. The method of claim 1 furthercomprising vacuum baking the lid substrate prior to attaching thegold/tin solder preform.
 4. The method of claim 1 wherein depositing theadhesion layer comprises depositing one or more materials selected fromthe group consisting of titanium, tantalum, and chromium.
 5. The methodof claim 1 further comprising tack welding the gold/tin solder preformto the solder base layer.
 6. The method of claim 1 wherein forming thelid substrate comprises one of stamping and drawing the lid substrate toone of a concave shape and a stepped shape.
 7. The method of claim 1further comprising depositing the adhesion layer onto opposing surfacesof the lid substrate.
 8. The method of claim 1 further comprisingforming the palladium solder base layer to surround all surfaces of thelid substrate.
 9. A method for making a non-magnetic hermetic packagecomprising: providing a package body comprising non-magnetic walls and ametallic non-magnetic seal ring disposed on a sealing edge of thenon-magnetic walls; bonding a component within an interior cavitydefined by the non-magnetic walls of the package body; providing ametallic non-magnetic lid comprising a gold/tin solder preform coupledto a lid substrate consisting of molybdenum, the metallic non-magneticlid further comprising an adhesion layer, a palladium base solder layer,and a copper seed layer positioned between the adhesion layer and thepalladium solder base layer; and seam sealing the gold/tin solderpreform to the metallic non-magnetic seal ring.
 10. The method of claim9 further comprising hermetically sealing the component within theinterior cavity.
 11. The method of claim 9 further comprisingfabricating the metallic non-magnetic lid by: shaping a blank consistingof molybdenum into the lid substrate; depositing the adhesion layer ontothe lid substrate; sputtering the copper seed layer onto the adhesionlayer; applying the palladium solder base layer onto the copper seedlayer via one of an electroplating technique and a sputtering technique;and tack welding the gold/tin solder preform to the palladium solderbase layer.
 12. The method of claim 9 further comprising applyingelectrical current to a surface of the lid substrate and to the metallicnon-magnetic seal ring to locally reflow the gold/tin solder preformtogether with the metallic non-magnetic seal ring.
 13. The method ofclaim 9 further comprising enclosing clean dry air within the interiorcavity.
 14. The method of claim 13 further comprising enclosing theclean dry air at a dew point less than about −40° C., a relativehumidity of 0%, and a particulate concentration of less than about10,000 ppm.
 15. The method of claim 9 further comprising seam sealingthe gold/tin solder preform to the metallic non-magnetic seal ringwithin a dry air glove box.
 16. The method of claim 9 further comprisinglocally heating the gold/tin solder preform to seam seal the metallicnon-magnetic lid to the metallic non-magnetic seal ring and create afillet extending up over an outer edge of the metallic non-magnetic lid.17. A method of manufacturing a metallic, non-magnetic lid that isseam-sealable onto a package, the method comprising: providing asubstrate consisting of molybdenum; and depositing an adhesion layer, acopper seed layer, and a palladium solder base layer sequentially ontoat least one surface of the substrate, wherein depositing each layer isaccomplished by using one of physical vapor deposition, chemical vapordeposition, and electroplating.
 18. The method of claim 17 furthercomprising attaching a gold/tin solder preform to the palladium solderbase layer.
 19. The method of claim 18 further comprising depositing thepalladium solder base layer to surround all surfaces of the substrate.20. The method of claim 18 further comprising vacuum baking the lidprior to attaching the gold/tin solder preform.
 21. The method of claim17 wherein depositing the adhesion layer comprises sputtering one oftitanium, tantalum, and chromium.
 22. The method of claim 17 furthercomprising depositing each of the adhesion layer, the copper seed layer,and the palladium solder base layer such that each layer surrounds thesubstrate.
 23. The method of claim 17 further comprising depositing agold strike layer between the copper seed layer and the palladium solderbase layer.